The present invention relates generally to an apparatus and method for the continuous casting of molten steel, and more particularly to the mold which is employed therein.
Continuous casting molds for molten steel are conventionally composed of copper or an alloy of copper. In conventional continuous casting, the mold has open upstream and downstream ends and side walls. Molten steel is introduced into the upstream end of the mold for flow in a downstream direction. The mold side walls contain the molten steel against flow in a direction transverse to the downstream direction.
The mold contains channels through which a cooling fluid is circulated. The cooling fluid carries away from the mold heat which is absorbed from the molten steel introduced into the interior of the mold, causing the molten steel to solidify as it moves in a downstream direction through the mold. Initially, a thin shell of solidified steel is formed adjacent the interior surface of the mold, and as the molten steel moves in a downstream direction through the mold, the shell of solidified steel thickens.
In another type of continuous casting, known as rheocasting or slurry casting, the mold is located downstream of a mixing chamber which receives molten steel and subjects the molten steel to vigorous stirring to break up solidifying dendrites which can form as the molten steel moves through the mixing chamber. A dendrite is a solidified particle shaped like an elongated stem having transverse branches extending therefrom. Breaking up the dendrites produces a material, for introduction into the casting mold, consisting primarily of molten steel together with a minor portion of fragmented and/or degenerate dendritic particles. A degenerate dendrite is a fragmented (broken up) dendrite having rounded off ends. Examples of rheocasting methods and apparatuses are described in the aforementioned Kelly, et al. U.S. Pat. No. 5,178,204.
Another type of continuous casting, known as continuous strip casting, employs a pair of opposed, counter-rotating, cooled rolls, in the case of double substrate continuous strip casting. The two counter-rotating rolls define the mold. There are a pair of side openings each at a respective opposite end of the pair of rolls. A containment dam is located at each side opening to prevent molten steel introduced between the rolls from flowing outwardly through the side opening. The molten steel is cooled as it descends downstream between the rolls, and a continuous strip of solidified steel exits the mold at the nip of the rolls, moving in a downstream direction.
In another form of continuous strip casting, called single substrate casting, a single roll rotates upwardly adjacent an open side of a tundish containing molten metal, and a strip solidifies on the periphery of the rotating roll. Containment dams are located at least on opposite sides of the roll at the junction of the roll and the ends of the adjacent side walls of the tundish.
In the case of a conventional continuous caster, it has been desirable to employ, in association with the casting mold, a system for magnetically stirring the molten steel within the casting mold; in the case of a rheocasting apparatus, such a system is essential. Typically, a current-conducting coil is located around the exterior of the casting mold, and a time-variable electric current is flowed through the coil which causes the coil to generate a magnetic field which is directed into the molten steel within the mold. This creates flow conditions within the molten steel in the mold which causes the molten steel to undergo stirring.
Either copper or an alloy of copper has been conventionally employed as the material of which a continuous casting mold is composed because copper has relatively high thermal conductivity which promotes rapid solidification of the steel. This increases the rate at which the steel can be continuously cast, and that is desirable. As used herein, the term "copper alloy" refers to those copper alloys heretofore conventionally used as the material for molds employed in the various types of continuous casting.
There is a drawback arising from the use of copper or copper alloy as the material of which the mold is composed. Because of its electrical conductivity (low electrical resistance), copper is relatively difficult for a magnetic field to penetrate. A substantial portion of the strength of the magnetic field (e.g. 50%) is attenuated due to the high electrical conductivity (low resistance) of copper or copper alloy.
It would be desirable to provide a continuous casting mold composed of a material which could remove heat from the molten steel contained within the mold at a rate approaching that of copper while avoiding the attenuating effect which copper has on a magnetic field.
In an apparatus for continuous strip casting, the rotating rolls are conventionally composed of copper, and in many embodiments of double substrate casting, magnetic containment dams are employed to generate a magnetic field extending across the side opening between counter-rotating rolls, to prevent the molten steel from flowing outwardly through the side opening. Examples of a double substrate continuous strip casting apparatus employing magnetic containment dams are disclosed in Praeg U.S. Pat. No. 4,936,374, in Lari, et al. U.S. Pat. No. 4,974,661 and in Gerber, et al. U.S. application Ser. No. 07/902,559, filed Jun. 22, 1992, In single substrate continuous strip casting, a magnetic containment dam, at the junction of the roll and the adjacent ends of the side walls of the tundish, prevents molten metal from flowing out of the tundish at that junction.
In continuous strip casting apparatuses, the copper, of which the rotating rolls are composed, has an attenuating effect on the strength of the magnetic field generated by the magnetic containment dam. As with molds employed in conventional continuous casting or in continuous rheocasting, it would be desirable to provide a mold for continuous strip casting which could extract heat from the molten steel at a rate approaching that of copper and which avoids the attenuation that copper has on the strength of the magnetic field.